Recording and reproduction of electrical impulses



Feb- 4, A. RINGEL 2,029,389

RECORDING AND REPRODUCTION OF ELECTRICAL IMPULSES Filed Feb. 1, 1935 2 sheets-sheet 1 j? i Y 'g Lf. 5 L/m airmail; ,fast/um 1E f7 h i it 7,9 fz f5 fo l B+ mrs/z ,wmf/m Z /4 flz/f/@e 55m/@ MDW/ff? ,29 f3@ ,3]

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RECORDING AND REPRODUCTION OF ELECTRICAL IMPULSES Filed Feb. 1, 1953 2 sheets-sheet 2 NEEDLE INVENTOR ABRAHA Pd GEL ATTORN EY Patented Feb.v 4, 1936 PATENT OFFICE f RECORDING AND REPRODUCTION OF ELECTRICAL IMPULsEs Abraham Ringel, Merchantville, N.J., assignor to Radio Corporation of America, a corporation of Delaware pplication February 1, 1933, Serial No. 654,667

21 Claims.

This invention relates to the recording and reproduction of electrical impulses such as those found within the audio frequencyrange, and has for its principal object the provision of an improved apparatus and method of operation where'- by ground or background noise is eliminated, the

volume range in sound recording and broadcasting is increased, and overshooting due to overy modulation of the sound during recording is prevented.

In the reproduction of sound, and particularly of music, two essential factors must be considered, namely, quality and quantity. In music these factors are referredto as tone color and"tone volume; in engineering they are associated with frequency response and amplitude, respectively. The performance of apparatus with regard to the reproduction of a wide frequency range has received a great deal of attention and considerable advance has been made in recording and amplifyingequipment. times commented upon and objected to the compression of volume range which has appeared to be necessary, because of apparatus limitations.

2 The lower limit of volume is governed by the soycalled noise level, while the upper limit is governed by the overload point of the equipment. In recording, this point is reached when one groove is over-cut' into the adjacent groove.

"3 At the present time, the permissible volume range in either broadcasting or disc recording is of the order of or 30 decibels (approximately to 1 in amplitude). From the standpoint of the musician, it would be highly desirable to have this range increased to 60 or rI0 db., to reproduce, for instance, the music of a symphony orchestra.

Such a range cannot be realized without resorting to some means of volume control additional to that inherent in the record or broadcasting system. This invention relates to a method of obtaining an increase in the existing volume range by the use of volume-controlling signal, referred to hereafter as the control-tone. With the v equipment described, the volume range in phono- 4" graph reproduction can be extended to at least Briefly, the broadening of the reproducible dynamic or volume range is accomplished in the following manner. Suppose that a passage is bethan the ground noise of the system. Heretofore, the gain of 'the recording amplifier was raised at this point to raise the level of the music to a value above that of the noise. In accordance with this invention, the recordist increases the gain in the same manner, but he also adds an additional sub-audible tone, having a diierent frequency such as 20 cycles, to the cutters input. When the record is later reproduced, the voltage 60 obtained from this additional control tone is used However, musicians have many' ing recorded, Whose minimum amplitude is lowerl to reduce the gain of the reproducing amplifier.

In this manner, the reproduced signal may be reduced very nearly to the original level, and at the same time the favorable signal to noise ratio, ob-

amplitude is, of course, determined by the reproducing equipment and may be made as high as desired so that the reproduced dynamic range may be made almost identical with the original. 10

As hereinafter explained, these various results are achieved by the utilization of a volume control current which has a frequency outside the frequency range of the desired signals, is recorded simultaneously with the signals on the same rec- 15 ord area, and has its level controlled either manually or automatically to maintain the desired signal at a continuously high level or within a com` paratively narrow range of levels. Thus in the case of sound recording and reproduction, the 20 control current may, for example, have a subaudible frequency and may have its level controlled to increase the volume of the audio impulses at low modulation and to decrease the volume of theseimpulses at high modulation, thus 25 maintaining the signal current within a comparatively narrow range of levels. During reproduction, the control current is separated from the signal current either electrically or electromechanically and is rectified and is utilized to re- 30 store the volume of the reproduced sound to its original value. It will be readily understood that such a control current may also be utilized in connection with radio broadcasting to correct inaccuracies due to defective rendition in the studio' 35 and the like.

Further objects of the invention are to increase the dynamic range of disc reproduction from approximately 25 db., as at present, to 50 db., to increase the signal to noise ratio in reproduction 40 to a more favorable value by permittingthe recording to be done at a high and more or less uniform level, and reducing this level to the original by means of the control tone, and to extend the volume range of sound lm reproduction or radio 4' broadcast transmission.

The invention will be better understood from the following description when considered in connection with the accompanying drawings and its scope will be pointed out in the appended claims.

Referring to the drawings:

Fig. l is a diagrammatic representation of a recording system involving the invention,

Fig. 2 is a diagrammatic representation of a reproducing system constructed in accordance with the invention,

Fig. 3 is a modified form of recording apparatus arranged for automatic volume control of the recorded sound,

Figs. 4, 5 and 6 are explanatory diagrams il- 60Y -cordance with the invention',

' of 20 cycles being suitable for this purpose.

Fig. 7 is a side view of an electrical reproducer which is operable electromechanically to separate the control and signal impulses-reproduced from a grooved disk or the like, A

Fig. 8 is a plan View of the part of "the reproducer which functions to repro uce thecontrol impulses recorded on a. grooved disk, and

Fig. 9 is an explanatory diagram relating to the operation of the composite reproducer of Figs. 7 and 8.

The recording system of Fig. 1 includes a microphone or pick-up device I0 from which electrical impulses are supplied to a recorder I I through 4a suitable channel including anamplifler I2, an

adjustable volume control potentiometer I3, a high pass filter and a recording amplifier I li. It will be apparent that the channel `2I3I4 may include additional elements such as a mixer and other usual elements utilized in a channel of this character, and that the recorder II may be terial.

1 [Control current produced by a generator I5 .is injected into the channel I4 through means including a potentiometer I6 which may be mechanically coupled to the potentiometer t3, a coupling tube Il, a transformer I 8 and a low pass filter I9. 't l Under these conditions-the signal and control currents are simultaneously recorded on the same recording area lor sound track. The levels or volumes of the control and signal currents may be varied simultaneously in the case of integral control of the potentiometers I3 and I6. If the signal currents are of audio frequency, the frequency of the control current may lie within the subaudible range, a frequency of the order If the original sound is too loud to be recorded or modulated, its volume is reduced and the volume .1 or amplitude of the control current is decreased corder, and increasing the volume of signal which l may be recorded.

It is desirable that about 90% of the soundy track area should be allowed for the sound record and that the remainder utilized to record the control current i. e. a maximum of about 90% of the sound track should be utilized for the loudest sound or the highest percentage modulation. The lowest recording level of the signal should not be less than 25 to 30 per cent of full scale. If the signal level-decreases below this range of values, the potentiometers I3 and I6 are operated to raise the levels of the signal and control currents to the proper Value.

During silent passages, the gain is increased by the potentiometers I3 and I6 to a point where the control current is recorded at maximum level so that the high amplitude of the control current will reduce the amplification of the system and thereby lower the ground noise levels in the silent passages during reproduction. When the level of the recorded control current is high, the

volume of the reproduced sound is low. When the recorded level of 'the control current is low, the volume of the reproduced sound is high. In-

termediate reproduced sound volumes in the t'rol current which occupies the remainder of 'the record and is superimposed on the sound record. In this manner, a voltage ratio of 30 to l is readily obtained for sound that would.

normally 'record less than 30 percent of full scale and a ratio of 10 to l is obtained for sound that would record more than percent or exceed full scale. This invention therefore makes it possible to obtain a voltage range of 30 3 10, or 900 to l, i. e., at least 60 decibels range without ground rioiseas compared with the 30 to 35 range heretofore possible.

The reproducing system of Fig. 2 includes a record 2li/and' a pick-up device 2| from which electrical impulses are supplied` to a loud speaker 284 through an amplifier 22, a coupling transformer 23, a pair of push-pull connected variable fttubes 2li-25; a transformer 26 and a power amplifier 2'I. A resistor 28 is connected in the common lead of the input circuits of the tubes 24-25. Volume control current is supplied to this resistor from the pick-up device 2|. the amplifier 22, a lter 29vfor separating the control current from the signal current, a rectifier 30 for producing a direct current which varies in accordance with the level of the control current and a filter 3I for smoothing out the rectified impulses.

As hereinafter explained in connection with Figures 7, 8 and 9, the function of separating the signal and control current impulses may, in the case of reproduction from a disk record, be performed by a composite pick-up device instead of the electrical filter 29.

It will be apparent that the rectified control current supplied to the resistor 28 which is connected in the input circuit of the vacuum tubes 2li-25 will vary the bias on the control grids and thus function to vary the amplification and volume of the reproduced sound, this volume being reduced when the level of the control current is high and being increased when the level of the control current is low. It should be noted that the variable fr tubes 24 and 25 may be of the type commonly having the designation UX235 or RCA-58. Y The power amplier 2l may be omitted if these tubes are designed to have sufficient output current, to operate the loud speaker 28.

Although the volume control of the recording system of Fig. 1 is effected by manual manipulation of the potentiometers I3 and I6, automatic means may be provided for producing this result. Thus in the recording system of Fig. 3, electrical impulses are supplied from' the pick-up device or microphone I0 to the recorder I I through a channel including the amplier I2, a transformer 35, a push-pull amplification stage 36-31 containing variable p. tubes, a transformer38, a high pass illter 39-40-4I, and the power amplifier I4. Volume control current is supplied from the generator I5 to the recorder II through a channel including ai transformer 42, a push-pull ampliilcation stage 43.-44 using variable a tubes, a transformer 45, a low pass' filter 46-41-48 and the power amplier I4.

Current for automatically controlling the levels or volumes ofA the signal and control currents as previously explained is derived from a secondary winding 50 of the transformer 35. It will be observed that current from this winding is supplied through an amplier 5l, a transformer 52, an amplifier 53, a transformer 54, a rectifier and a lter 51-58--59 to a resistor 60 connected in the input or control circuit of the ampliiier 36-31 of the signal channel, and to a resistor 6| connected in the control circuit of the amplifier 43-44 of the control channel.

It is apparent that proper adjustment of the input potentiometers and ll will result in automatic variation of the control and signal current levels in the manner explained in connection with the system of Fig. 1. Thus when the signal current level is low, the voltage drops across the resistors 60 and 6I will be comparatively low and the signal and control current levels will increase. When the signal currentv level is high, however, the voltage drops of the resistors 60 and 6I will be comparatively high and the signal and control current levels will decrease. When no signal current exists, the voltage drop of the resistor 6l is very low and the level of the control current has its maximum value.

If the recorder is of the photographic type, the sound record appears somewhat as indicated in Fig. 4, the control current being indicated as a sine curve 62 on which are superimposed the signal waves 63. In the short record illustrated it is of course impossible to show variations in level. In the case-of the grooved record of Fig. 5, the audio waves are likewisesuperimposed on a groove of sinusoidal form. In Fig. 6, the signal current is roughly indicated by the curve A, the control current by the curve B, and the resultant current by the curve C.

The electrical reproducer illustrated by Figs. 7 and 8 includes an electrical reproducer 10 provided with a support or tone arm 1l pivoted at 12. This reproducer 1|l reproduces signal current from the record 13 in the usual manner and its operation will be readily understood without further explanation.

Mounted upon the tone arm 1l by means including a bolt 13 and a spacer 14 is a reproducer l5 which functions to reproduce the control current impulse recorded on the record 13. This control current reproducer includes a magnet 16 which is provided with an armature 11 mounted on a .resilient support 18 and having an adjustable weight or inertia means 19 for tuning the reproducer to the control frequency. Mounted on the armature 11 near the pole pieces of the magnet 1E is a pick-up coil 80 wherein the control current is generated.

The manner in which the electrical pick-up vdevice 15 is tuned by adiustment of the weight ma=moment of inertia of the tone arm 1l spond to the "control current frequency utilized.

As will be readily understood by those skilled in the art, this type of vfiltering is of particular advantage when a control current of relatively low frequency is utilized. How the control current thus separated from the signal is utilized to control the volume of the reproduced sound will be evident in view of what has been said with respect to Fig. 2.

From the foregoing explanation, it will be evident that the invention is not limited to the par'- ticular embodiment described but that various changes may be made without surrendering any of the advantages which it has made possible.

I claim:

1. The method of sound recording and reproducing whichv includes superimposing on the audio impulses a control current of inaudible frequency, raising the level oi said control current when the volume of said impulses is relatively low and lowering said level when said volume is relatively high, reproducing said current and impulses, `*and controlling the volume of said reproduced impulses in accordance with said reproduced current.

2. The method of sound recording and reproducing whichl includes superimposing on the audio impulses a control current of inaudible frequency, raising the level of said control current when the volume of said impulses falls below thirty per cent of its full scale value and lowering said level when said volume exceeds ninety per cent of its full 'scale value, reproducing said current and impulses, and controlling the volume of said reproduced impulses by means of said reproduced current.

3. The method of sound recording and reproduction which includes superimposing on the audio impulses a control current of inaudible frequency, raising the level of said control current when the range of said volume is relatively low and lowering said level when said range is relatively high, reproducing said impulses and current, and controlling the volume of said reproduced impulses by means of said reproduced current.

4. The method of sound recording and reproducing which includes superimposing on the audio impulses to be recorded a control current of inaudible frequency, raising the levels of said impulses and current during the lower volume ranges of said sound and lowering said levels during the higher volume ranges of said sound,

l reproducing said impulsesand current, and restoring the level of said reproduced impulses to its original value by means of said reproduced current.

5. The method of sound recording and reprovso and controlling the volume of said reproduced impulses by means of said reproduced current.

6. The method which includes producing audio impulses, superimposing a current of inaudible frequency on said impulses, reproducing said impulses and current, and controlling the volume of said reproduced impulses by means of said reproduced current.

7. 'Ihe combination ofmeans for subjecting a light sensitive record to light modulated in accordance with electrical impulses, means for superimposing on said impulses additional impulses of a frequency without the range of said modulatingimpulses, and means for. simultaneously varying the levels of said impulses.

8. The combination of means for subjecting a light sensitive record to light modulated in accordance with electrical impulses, means for superimposing on said impulses additional impulses of a frequency without the range of said modulating impulses, means for simultaneously varying the levels of said impulses, means for v reproducing said impulses, and means for causing the volume of said reproduced modulated impulses to be controlled in accordance with the level of said reproduced additional impulses.

9. 'Ihe combination of means for subjecting a record area to impulses within one range of frequencies and to an impulse of a frequency without said range, means for reproducing said impulses from said record, and means for causing the volume of the impulses reproduced within said range to be controlled by the impulse reproduced without saidrrange.

l0. The combination of means for producing audio impulses, means for producing an inaudible impulse, and means responsive to said audio impulses for controlling the levels of said audible and inaudible impulses. I

1l. The combination of means -for producing audio impulses, means for producing an inaudible impulse, means responsive to said audio impulses for controlling the levels of said audible and inaudible impulses, and means for causing said audible and inaudible impulses to be recorded in superimposed relation with respect to one another.

l2. The combination of means for producing audio impulses, means for producing an inaudible impulse, means responsive to said audio impulses for controlling the levels of said audible and inaudible impulses, means for causing said audible and inaudible impulses to bie/recorded in superimposed relation with respect to one another, means for reproducing said audio and inaubile impulses, and -means for causing the volume of said reproduced audio impulses to be controlled by said reproduced impulse of inaudible frequency. 1

13. 'I'he method of sound recording and reproducing which includes superimposing on the audio impulses a control lcurrent of inaudible frequency, lowering the level of said control current when the volume of said impulses is rela- Vtively high, reproducing said current and impulses', and controlling the volume of said reproduced impulses in accordance with said-reproduced current'.

14. The method of sound recording and reproducing` which includes superimposing on the audio impulses a control current of inaudible frequency, raising' the level of said control current when thevolume of said impulses falls below thirty percent of its full scale value, yreproducing said current and impulses, and controlling the volume of said reproduced impulses by means of said reproduced current.

15. The method of sound recording and lreproduction which includes superimposing on the audio impulses a control current of inaudible frequency, rais ng the level of said control current when the range of said volume is relatively low, reproducing said impulses and current, and controlling the volume of said reproduced impulses by means of said reproduced current.

16. The method of sound recording and reproducing which includes superimposing on the audio impulses to be recorded a control current of inaudible frequency, raising the levels of said impulses and current during the lwer volume ranges of said sound, reproducingv said impulses and current, and restoring the level of said reproduced impulses to its original value' by means of said reproduced current.

17. The combination of means for subjecting a record area to impulses within one range of frequencies and to an impulse of a frequency without said range, means for reproducing said impulses from said record, means for separating said reproduced impulse from said reproduced impulses, and means for causing the volume of said reproduced impulse to be controlled by said reproduced impulse.

18. The combination of means for subjecting a record area to impulses within one range of frequencies and to an impulse of a frequency without said range, means for reproducing said impulses from said record, electrical means for separating said reproduced impulse from said reproduced impulses, and means for causing the volume of said reproduced impulses to be controlled by said reproduced impulse.

19. The combination of means for subjecting a record area to impulses Within one range of frequencies and to an impulse of a frequency without said range, means for reproducing said impulses from said record, mechanical means for separating said reproduced impulse from said reproduced impulses, and means for causing the volume of said reproduced impulses to be controlled by said reproduced impulse.

20. The combination of means for subjecting a record area to impulses Within one range of frequencies and toNan impulse of a frequency without said range, means for reproducing said impulses from said record, means including a composite reproducer for separating said reproduced impulse from said reproduced impulses, and

means for causing the volume of said reproduced impulses to be controlled by said reproduced impulse.

21. Apparatus comprising a pickup device,

means operatively associated with the pickup defor actuating the control means in response to control vibrations made eifective in the pickup device.

ABRAHAM RINGEL 

